OC­TANE AND AL­TI­TUDE

Car (South Africa) - - TECH -

NAT­U­RALLY AS­PI­RATED EN­GINES

At­mo­spheric pres­sure has a sig­nif­i­cant im­pact on the in-cylin­der pres­sures of nat­u­rally as­pi­rated en­gines dur­ing WOT. As Jo­han­nes­burg, for ex­am­ple, is roughly 1 700 me­tres above sea level, the at­mo­spheric pres­sure is gen­er­ally 17% lower than at the coast, ex­plain­ing the drop in en­gine per­for­mance. This fact low­ers the ten­dency of the en­gine to knock, al­low­ing a lower-oc­tane fuel to be used with no loss in per­for­mance compared with a higher-oc­tane fuel be­cause the ad­di­tional ben­e­fit can­not be re­alised.

This was con­firmed in a study by Pro­fes­sor Andy Yates, who con­ducted ac­tual WOT per­for­mance test­ing on a fleet of 18 ve­hi­cles in Gaut­eng us­ing vary­ing oc­tane-rated petrol. He found the fol­low­ing:

The top graph de­picts the prob­a­bil­ity of max­i­mum per­for­mance gain by oc­tane num­ber at al­ti­tude. Most man­u­fac­tur­ers cal­i­brate their petrol en­gines to run op­ti­mally on 95 oc­tane at the coast, although some gains are still pos­si­ble. In­ter­est­ingly, 87-oc­tane petrol ap­pears to sat­isfy most en­gines’ max­i­mum per­for­mance po­ten­tial at al­ti­tude and is con­firmed by the se­cond graph show­ing no ben­e­fit at al­ti­tude to use an oc­tane rat­ing higher than 91. Stud­ies show, with each in­crease of 300 me­tres in al­ti­tude, drops in oc­tane rat­ing of be­tween 1,0 and 1,8 can be safely used with no neg­a­tive ef­fect on per­for­mance from a Mbt-tim­ing point of view.

Ba­sis: SAE pa­per 2003-01-2012, Fleet Tests to De­ter­mine the Oc­tane Re­sponse at Di er­ent Al­ti­tudes for Ve­hi­cles Equipped with Knock Sen­sors

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